1. Field of the Invention
The invention relates generally to guidewires for directing catheters or other medical instruments through the cardiovascular system.
2. Background Information
Guidewires for use in, for example, percutaneous transluminal coronary artery angioplasty (PTCA), must be thin and flexible enough to advance through small arteries within the coronary vasculature. These wires must also be sturdy enough to be manipulated from the outside of the body, such that a distal end of the wire can be brought into contact with a selected region of the coronary artery. Further, they must be strong enough to survive a xe2x80x9cpull testxe2x80x9d without breaking, to ensure that they do not come apart in the body.
Numerous guidewire designs exist. These designs have typically been made from stainless steel materials and may have platinum coils added to increase radiopacity. Coatings such as PTFE, silicon, and hydrophilic materials may be added to reduce friction and improve movement of devices that are passed over the guidewire.
Stainless steel guidewires are inherently stiff and offer excellent support along the proximal shaft portion of the guidewire. The distal ends of these wires may also be deliberately bent or shaped to aid in steering the guidewire into a particular vessel or lumen. The material, however, is susceptible to further plastic deformation during use and has been known to permanently deform and kink. The deformation is particularly noticeable when the guidewire is manipulated through a tortuous anatomy.
More recently, guidewires have been made from xe2x80x9csuper elasticxe2x80x9d materials such as Nitinol, with coils added for radiopacity and coatings for lubricity, as mentioned above. The super elastic guidewires offer excellent kink resistance, and provide exceptional torque control when placed within tortuous anatomies. The super elastic material, however, is significantly less stiff than stainless steel and therefore does not provide a high level of support along the proximal portion of the guidewire. Further, the super elastic material cannot be easily shaped or reshaped at the distal tip to aid in steering the guidewire.
Composite construction guidewires combine a proximal portion of stainless steel with a distal portion of super elastic material, to take advantage of the best performance characteristics of both materials. Unfortunately, it is very difficult to attach non-super elastic materials to super elastic materials. The joint cannot, for example, be held together by braising or welding. Accordingly, a special coupling must be used to lock the materials together.
In a known prior system the ends of the two guidewire portions, that is, the ends of the two materials, are butted against one another and a sleeve, which made of non-super elastic material, is fit over the joint. The guidewire portions are then held together by crimping, spot welding or gluing the sleeve in place. The coupling relies mainly on the mechanical interface between the two portions of the guidewire. If a mismatch in the cross-sectional dimensions exists, the distal and proximal portions may separate. Further, the repeated torqueing and bending that occurs when the guidewire is manipulated through the cardiovascular system may fatigue the coupling and result in the separation of the distal and proximal portions of the guidewire within the patient""s body.
A composite guidewire constructed in accordance with the invention includes a solid central core that is made out of super elastic material. A coupling tube which is usually of a non-super elastic material fits over the proximal end of the central core, and a coil, which may be radiopaque, fits over the distal end of the super elastic central core and attaches to a distal end of the coupling tube. A flat safety wire or ribbon of a non-super elastic material that is positioned between the distal portion of the coupling tube and the central core also attaches to the distal end of the coupling tube. The safety wire extends the length of the coil and attaches to both the distal and the proximal ends of the coil. The coil, the coupling tube and the safety wire attach to one another by soldering, brazing, welding or adhesives, which ensures that the joints are strong and the various components of the guidewire do not pull apart. The tube may also be crimped at various locations along its length, to prevent rotational and axial movement between the non-super elastic tube and the super elastic core. An atraumatic tip fits over the end of the guidewire and attaches to the coil and the safety wire by brazing welding or adhesive, to provide a cushioned end.
The coil and the safety wire may extend beyond the distal end of the central core, such that the distal end of the guidewire can be readily shaped for steering.
The guidewire may also be formed with a composite core that includes the super elastic core and a non-super elastic core extension, which is shaped at its distal end to overlap and mechanically inter-lock with a proximal end of the super elastic core. The coupling tube then extends partially over a portion of the non-super elastic core extension and partially over a portion of the super elastic core, with the proximal end of the coupling tube attaching to the core extension by brazing, welding or adhesives. The coil that fits over the distal end of the super elastic core mates at its proximal end with the distal end of the coupling tube. The safety wire runs from the end of the core extension to distal end of the guidewire, and attaches to the core extension, the coupling tube, the coil and the tip.
In another variation of the guidewire, instead of interlocking the opposing ends of the super elastic core and the non-super elastic core extension, those ends may be butted and the core and core extension provided with longitudinal flats so that the safety wire can extend lengthwise between those elements and the coupling tube. Then the remaining radial space between the core-core extension and the coupling tube is filled with epoxy resin. The epoxy coupled with the irregular cross-sections of the core-to-core extension assembly creates a strong joint able to transmit considerable torque without failure.
The various components of the composite guidewire embodiments attach to components of like or similar materials, such that the joints between the components can be formed by brazing, welding or adhesives. The joints are thus strong, and do not fatigue as readily as joints between the dissimilar non-super elastic and super elastic materials that occur in known prior composite guide wires. The guidewire, with its super elastic core, takes advantage of the associated resistance to kinking and torque control. Further, the coupling tube overlaid on the super elastic core provides both support for the proximal portion of the guidewire and the ability to shape the distal end of the guidewire.